Abstract 3412: Cis demasking cytokine technology improves therapeutic index of immunocytokine

Abstract Immunocytokines are promising therapeutic approaches to enhance T cell response by delivering interleukins directly to the tumor site. The fusion of wild-type or attenuated interleukins with anti-PD-1 antibodies has shown some efficacy in preferentially cis-activating PD1+ T-cells in tumor and lymph nodes. However, for many immunocytokines (e.g. IL-2/IL-15/IL-21/IL-12), off-target activation of PD1-negative cells remains a significant challenge due to the high affinity of the cytokine for their receptors, leading to unwanted systemic effects, toxicity, high clearance and low tumor biodistribution, which ultimately limits their clinical potentials. To address this low therapeutic index issue, strategies have emerged to achieve localized cytokine activation using conditional masking strategies (e.g. Enzymatic cleavable linkers) but their clinical translation remains challenging due to off-tumor activation. To overcome these limitations, we developed OSE-Cytomask® Platform, a universal and non-cleavable linker technology allowing cytokine CIS-demasking upon binding of the fused antibody to its target. This platform enables precise cytokine activation on the right T cell population to minimize toxicity and improve therapeutic index of immunocytokine. A series of linkers were screened for optimal cis activation of the cytokine on PD1+ vs PD1neg T cells fused to a high-affinity anti-PD1 antibody. OSE-Cytomask® linker technology strongly decreases IL-2 or IL-15 cytokine activity on PD1neg cells while maintaining strong activation of PD1+ T cells even in a co-culture system. We confirmed OSE-Cytomask® activity on PD1+ primary T cells while very low activation has been observed in naïve T cells. OSE-Cytomask® linker technology does not induce TRANS-activation on PD1neg T-cells illustrating a strict CIS-demasking activity. Importantly, Anti-PD1/IL-15 and anti-PD1/IL-2 OSE-Cytomask® technology strongly reduced toxicity of the immunocytokine while maintaining in vivo anti-tumor in vivo efficacy. At least 10-fold higher therapeutic index for OSE-Cytomask® Anti PD1/IL2v was obtained compared to the conventional Anti PD1/IL2v. In conclusion, OSE-Cytomask® linker technology illustrates specific intrinsic property to mask cytokine on naïve peripheral immune cells not expressing the target of the antibody while allowing selective CIS-demasking and activation of the cytokine on PD1+ cells. Unlike protease-cleavable linker technologies, OSE-Cytomask® does not rely on enzymatic activity, making it a more stable and adaptable solution across different TME conditions. It has been validated with multiple cytokines (6 tested) and can be fused with various antibodies. By reducing off-tumor cytokine activity and selectively activating immune cells, OSE-Cytomask® platform offers a promising strategy for the clinical development of more effective and safer immunocytokines for cancer therapies. Citation Format: Caroline Mary, Margaux Seite, Virginie Thepenier, Geraldine teppaz, Justine Durand, Kévin Biteau, Cécile Batty, Ariane Desselle, Julien Taurelle, Virginie Aillerie, Nicolas Poirier, Aurore Morello. Cis demasking cytokine technology improves therapeutic index of immunocytokine [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2025; Part 1 (Regular Abstracts); 2025 Apr 25-30; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2025;85(8_Suppl_1):Abstract nr 3412.